A widely used form of packaging for electronic devices such as IC devices is a plastic housing. Typically, IC chips are bonded to a substrate and a polymer is molded over the assembly to overmold the device. It is common for two or more IC chips to be assembled in a single overmolded package. Multiple chip packages are referred to as multi-chip-modules (MCMs).
As chip size decreases in state of the art IC technology, the problem of overheating in IC packages becomes more severe. It is further aggravated because polymers used for overmolding are poor thermal conductors. Thus while the plastic effectively encapsulates the devices, it traps the heat generated by the devices as well. In packages in which the IC chip is connected to the electrical terminations of the package with wire bonds, the thickness of the encapsulant must be sufficient to accommodate the height of the wire bonds. This results in a thick “cover” of plastic over the device. Since the thermal resistance of any given material decreases with increasing thickness, increased thickness further retards heat dissipation, all else being constant.
A wide variety of heat sink expedients have been proposed and used to address thermal management issues. Among these, and tailored for the types of packages with wire bonded IC chips, is the use of a conductive “chimney” that attaches to the top of an IC chip and becomes imbedded in the plastic overmold. The conductive chimney conducts heat away from the IC chip, through the thickness of the plastic overmold but through the chimney itself and not the plastic overmolded material to the top of the package. In some package designs, the top of the chimney is affixed to a lid. The lid may be made of metal, which effectively spreads the heat and conducts the heat to the external environment. In conventional designs, the chimney is attached to the lid using a thermal interface material (TIM). While any heat conductive material may be used for the chimney structure, silicon is preferred because of its thermo-mechanical compatibility with the silicon chips, low cost, availability, compatibility wih existing IC assembly equipment, and good thermal conductivity.
Device failures have been identified in these package designs. Improvement in the package design is needed to overcome these failures.